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Infectious Disease: Question 5

Syllabus 10.2

Multiple choice AS 1 mark

For several years, doctors in a hospital ward used the same antibiotic to treat patients infected with Staphylococcus aureus. Initially, almost all Staphylococcus aureus bacteria in the ward were killed by this antibiotic. After several years of frequent use, most Staphylococcus aureus isolated from patients in the ward were resistant to it, a strain known as MRSA (methicillin-resistant Staphylococcus aureus).

Which statement correctly explains how this increase in antibiotic resistance came about?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall how natural selection produces antibiotic resistance

Random mutations occur in bacterial DNA independently of whether an antibiotic is present. In a large population of Staphylococcus aureus, a small number of bacteria may already carry a mutation that makes them resistant to a particular antibiotic, purely by chance, before that antibiotic is ever used against them.

When the antibiotic is introduced, it acts as a selection pressure: non-resistant bacteria are killed, while the rare resistant bacteria survive. These survivors are then free to reproduce without competition from the non-resistant bacteria, passing the resistance allele on to their offspring. Repeated or frequent use of the antibiotic keeps favouring resistant bacteria over non-resistant ones, so the proportion of resistant bacteria in the population (here, MRSA) increases generation after generation until it dominates the population.

Step 2: Evaluate each option

  • A: incorrect. This describes the antibiotic inducing new mutations, but mutations arise randomly and are not caused by the antibiotic itself; the antibiotic only selects between variants that already exist.
  • B: correct (this describes pre-existing genetic variation, selection (survival of resistant bacteria, death of non-resistant bacteria), and increasing allele frequency through reproduction) the correct sequence for natural selection.
  • C: incorrect. This describes bacteria acquiring a characteristic during their lifetime and passing it on, which is not how inheritance works; only bacteria that already carried the resistance allele can pass it to offspring.
  • D: incorrect, this suggests a change in the antibiotic itself affecting all bacteria equally, rather than a genetic difference between individual bacteria determining which survive.

Final answer

  • The increase in resistance is explained by natural selection acting on pre-existing genetic variation: resistant bacteria already present survived the antibiotic and reproduced, increasing the frequency of resistance in the population, option B.